As a technical reference document filed by the present applicant, for example, Patent Document 1 (JP 10-235720 A) discloses a technology of molding a panel with skin by blow molding a cylindrically shaped molten resin.
According to Patent Document 1, a panel with skin is to be molded by thermally welding a skin material to the outer surface of one wall under the blow pressure during blow molding, and forming, on the other wall, inner ribs that are caused to protrude until the inner ribs are brought into contact with the aforementioned wall.
However, in a case in which a cylindrically shaped molten resin is used as in the case of Patent Document 1, since the molten resin at the part where the inner ribs are to be formed is stretched out, it is necessary to make the wall thickness of the cylindrically shaped molten resin thick so that pinholes are not generated. As a result, there is a problem that the weight of the panel with skin to be finally molded becomes heavy. Therefore, in a case in which a cylindrically shaped molten resin is used as in the case of Patent Document 1, there is a problem that it is difficult to promote the weight reduction of the resin panel to be finally molded.
Usually, the thickness (wall thickness) of a cylindrically shaped molten resin (cylindrical parison) is made uniform. Also, when parts of a split mold are clamped, the pressing force of pressing the cylindrical parison against the split mold becomes even over the entire surface of the cylindrical parison. Therefore, the parison that is pressed against one part of the split mold, on which ribs such as inner ribs are formed, is stretched out in accordance with the blow ratio set in relation to the ribs, and local thin portions are produced in the finally molded resin panel on the side that is pressed against this part of the split mold. On the other hand, thin portions are not produced on the side of the panel that is pressed against the part of the split mold without ribs. As a result, it is necessary to set the thickness of the cylindrical parison in accordance with the thin portion generated on the one part of the split mold on which ribs are formed, and this causes the panel thicker than required that is produced on the other part of the split mold. Therefore, in a case in which a cylindrical parison necessarily having a uniform thickness is used, a problem arises that it is difficult to reduce the weight of the finally molded resin panel, because the wall on which ribs are not formed after blow molding becomes thicker than the wall on which ribs are formed.
Furthermore, in the case of using a cylindrically shaped molten resin, one wall and the other wall are constructed from the same molten resin material. Therefore, there is also a problem that it is difficult to partially enhance the impact resistance only by means of the resin material that constitutes the one wall and the other wall. Given such a situation, it is necessary to achieve weight reduction of the resin panel and to obtain a resin panel with enhanced impact resistance.
Meanwhile, in order to promote weight reduction of a resin panel, for example, the weight reduction can be realized by using a pair of thermoplastic resin sheets extruded from an extrusion apparatus as in the case of Patent Document 2 (JP 2010-201662 A). According to Patent Document 2, a pair of thermoplastic resin sheets extruded from an extrusion apparatus is disposed in a pair of split mold parts. Therefore, when the wall thickness of the thermoplastic resin that is extruded into the split mold part on which ribs are formed, and the wall thickness of the thermoplastic resin that is extruded into the other split mold part on which ribs are not formed are separately adjusted, and when causing the wall thickness of the thermoplastic resin that is extruded into the other split mold part on which ribs are not formed, to be unnecessarily thick is avoided, the weight reduction of the resin panel to be finally molded can be promoted.